1. Field of the Invention
The present invention relates generally to charge-transfer-device structures and more particularly to an improved input circuit for inserting charge packets into a charge-transfer-device structure.
2. Description of the Prior Art
A number of techniques for inserting charge packets into a charge-transfer-device are known in the prior art. A review of some of these techniques are set forth in the publication entitled "Use of Charge-Coupled Devices For Delaying Analog Signals," Michael F. Tompsett and Edward J. Zimany, Jr., IEEE Journal of Solid State Circuits, Vol. SC-8, No. 2, Apr. 1973.
In this publication it is stated that in one injection technique the input signal is capacitively coupled to the input diode of the charge-coupled device and the input gate is held positively biased. When the first transfer electrode is pulsed on, a potential well, or virtual drain is created with respect to the input diode or source. A current that is dependent on the potential of the input diode then flows into the well from the diode. When the first transfer electrode turns off, the current flow stops and the generated charge packet transfers along the device. In this mode of controlling the input of charge into the device, the signal may be applied either to the input diode, as indicated already, or the diode may be held at a fixed bias and the signal applied to the input gate. An alternative method is to let the input signal modulate an external current source and apply the current to the input diode.
Another way to control the size of the charge packets is to set the surface potential of the inverted region under the first transfer electrode when charge has come to equilibrium. This may be done by strongly biasing the input gate on and applying the signal to the input diode. The following sequence may be used. The input signal is applied to the input gate and the input diode is pulsed each cycle to a low value so as to completely fill the region under the first transfer electrode with charge. The input diode is returned to a strongly reverse biased state. Excess charge then flows from the region under the first transfer electrode until the potential of this region equals the potential under the input gate. Hence the potential under the first transfer electrode containing the charge packet and therefore the amount of charge in the packet is controlled by the difference potential in the input gate and the first electrode and is independent of the flatband voltage.
In all these methods, charge samples dependent on the input signal can be formed and passed along the device. In addition, the signal level may be adjusted so that small background or bias charge packets are obtained even when the analog signal is zero. This is necessary so as to keep the interface states filled in the absence of signal and to obtain the best transfer efficiency from the device.
The choice of a particular method to input charge into a CCD would normally be made on the basis of maximizing the linearity and reducing harmonic distortion in the delayed signal. However, all methods known heretofore in the prior art have some factor causing nonlinearity. The present invention, on the other hand, provides a technique which does not depend on threshold and is therefore essentially distortionless and precise.